Petrophysical Training

Petrophysical Consulting

LITHOLOGY
BASICSIn
oil field applications of logs, interest is primarily directed
to definition of the amount and type of fluids in the formations.
These determinations require that matrix effects be defined and
accounted for through appropriate assumptions about the mineralogy
of the reservoir or by combinations of logging measurements that
automatically compensate for mineral effects. In addition, we
have found that a knowledge of the mineral composition of the
reservoir aids in understanding its depositional environment, porosity
distribution, production characteristics, and exploitation potential.
So lithology or mineralogy results from petrophysical analysis are
worthwhile pursuits in their own right.

In
coal, evaporite, and mineral exploration, the primary interest
is in the identification of the minerals - porosity is usually
negligible or less important. Mathematically, the oil-field and mining situations
are identical, so the methods described here apply to both disciplines
equally.

Before
proceeding, we need to define the nature of rocks more clearly.

An element is a primary component of a chemical compound.
Familiar elements are iron (Fe), calcium (Ca), carbon (C), and
oxygen (O).

A
mineral is a naturally occurring inorganic compound with a specific chemical
formula and a defined crystal structure. Many naturally occurring
minerals are impure, so their chemical makeup varies slightly.
Familiar mineral compounds are quartz (SiO2) and calcite (CaCO3).

A rock is made from a mixture of minerals, although one
mineral may dominate. For example, some sandstones are mostly quartz
(SiO2) but many other minerals may also be present. Other sandstones
may be mostly feldspar with little quartz. Limestone is a rock
containing mostly calcite (CaCO3) but other minerals may be mixed
with it. Most rocks have a wide range of minerals and the fraction
of each mineral in a rock may vary widely from one sample to
another.

Minerals are often described and identified
by, their hardness, magnetic response, colour, luster, streak,
cleavage, crystal form, specific gravity, reaction to acid, or even
their taste and smell. These terms are useless for petrophysical log
analysis, which relies on physical properties that can be measured
remotely in a well bore, such as density, acoustic velocity, neutron
and gamma ray response, or electrical resistivity.

Minerals are classified into groups and sub-groups. Silicate minerals have silicon and oxygen in their composition. There are four
types of silicate minerals:

Single chain
silicate (eg. augite)

Double chain
silicate (eg. hornblende)

Sheet silicate
(eg. micas and clays)

3-D framework
silicate (eg. feldspars, quartz)

Silicates are also divided into two groups based on their
color and density. Light (nonferromangesian) silicates are light in
color and have a specific gravity around 2.7. Light silicates
contain various amounts of aluminum, potassium, calcium and sodium.
Dark (ferromagnesian) silicates are dark in color and have a
specific gravity ranging from about 3.2 to 3.6. They contain mostly
iron and magnesium.

All other minerals are put into the non-silicate group, then
broken down into six subgroups:

Carbonates -
minerals that contain carbon and oxygen

Oxides -
minerals with an oxygen base

Sulfides -
minerals that contain sulfur

Sulfates -
minerals that contain sulfur and oxygen

Halides -
minerals that contain a metal and a halogen element

Native metals -
copper, silver, gold, zinc, iron, and nickel

ROCK
CLASSIFICATIONBecause
the earth has an active surface, minerals (in the form of rocks) are
under constant change. Molten rock from the interior of the earth
can be exposed at the surface from volcanoes or mid-ocean ridges.
When molten these rocks are called lava flows and when cool they are
called igneous rocks.

As
igneous rocks are eroded by weather and water, they become loose
grains or dissolved in water. When deposited, they become soil or
sediment, and later under the pressure of overburden, turn into
sedimentary rocks.

If
sedimentary rocks are forced deep enough, heat and pressure modify
the rock structure. These are called metamorphic rocks.

All
three kinds of rocks can contain porosity that can hold economic
quantities of oil and gas, although sedimentary reservoirs are much
more common. Any of these rock types can re-enter the mantle and
become molten again, by subduction at the edges of tectonic plates.
This cycle of igneous – sedimentary – metamorphic is called the rock
cycle.

Sedimentary rocks are an accumulation of fragments of many pre-existing rocks. Weathering
is a process by which rocks are broken down into sediments. There
are two types of weathering:

Mechanical -
weathering in which physical process such as frost wedging and
unloading break down rocks.

Chemical -
weathering in which chemical processes such as oxidation break
down rocks.

Transport describes the process by which sediments are moved
across the surface. Types of transport include fluvial, glaciers,
wind (aolean), and gravity.

Depositional environments describes where sediment comes to
rest, The three main groups however are:

Continental -
deserts, lakes, river beds, swamps, and caves

Continental and
Marine - deltas

Marine - ocean

Lithification is the process by which sediments come together to form a sedimentary
rock. There are three ways in which this is done:

Compaction –
the intense weight and compression caused by the weight of
overburden welds sediments together to form a sedimentary rock

Texture of a rock is based on the size, shape, and arrangement of the grains and
other parts of the rock. Sedimentary rocks can be broken down into
five different textures:

Clastic -
consists of broken fragments of pre-existing rock.

Bioclastic
- consists of the remains of organic material.

Crystalline
(Nonclastic) - minerals are in a pattern of interlocking
crystals.

Amorphous -
no crystal structure .

Oolitic -
made of small round particles of calcium carbonate.

Mineral
composition in sedimentary rocks varies widely.

Silicates

Carbonates

Clay Minerals

Organic Matter

Evaporites

(Volcanic) Rock
Particles

Heavy Minerals

Feldspar

Many
descriptive terms are used to define rock samples, most cannot be
determined directly from petrophysical logs. Shape, sorting, bedding
type and bed thickness are common terms. Size of the sedimentary
particles is a semi-quantitative approach to sample description and
assists the petrophysicist in understanding the rock texture. Terms
used are:

The kinds of rocks we can identify with well logs depend on the logging tools
that have been run in the well bore,
the rock mixtures present, and local zone knowledge. In clastic
and carbonate sections, we can usually identify quartz, shale,
limestone, dolomite, anhydrite, coal, pyrite or glauconite or
siderite or
other heavy minerals, salt, potash, trona, sulphur, gypsum, and
a few rarer minerals like fluorite or barite, provided the minerals
occur as mixtures of only a few components and we have a full
modern log suite.

Shale minerals, such as montmorillonite, illite,
and chlorite, can be distinguished if we have additional logs.
Kaolinite and feldspars can also be defined under certain conditions,
as can mica. Although not discussed in this Chapter, hardrock
minerals and uranium deposits can be evaluated with well logs.

The
mineralogy of unconventional reservoir rocks, such as granite,
metamorphic, and volcanic rocks, can be evaluated with the techniques
described here, provided the list of minerals is small and their
physical properties can be determined.

In
most carbonate reservoirs, the lithology is usually reasonably
well known from sample descriptions or can be determined from
log response. This is not true in sandstones because the mineral
makeup of the sand is NOT usually described in much detail. There
is a universal trend to give sandstones the physical properties
of pure quartz, but this is almost universally NOT appropriate.
Most sandstones contain other minerals such as mica, volcanic
rock fragments, calcite, dolomite, anhydrite, and ferrous minerals,
as well as the shale and clay described above. All of these minerals
have different density, acoustic, and neutron properties than
quartz. If a sandstone is assumed to be pure quartz when it is
not, the commonly used properties of quartz will provide pessimistic
porosity answers.

Thus,
authors and service company manuals that present mineral properties
for “sandstone” are misleading their audience into
believing these properties are constant. In more than 40 years
of petrophysical analysis, I have never seen a thin section or
XRD report that gave an assay of 100% quartz in any petroleum
reservoir. A 100% quartz sand is very rare. If anyone doubts this
statement, look at the PEF curve. If it reads more than 1.8, you
have “quartz plus other things” in your sandstone.

There
is a story (it may even be true) that reserves for the early North
Sea discoveries were seriously underestimated because the mica
in the sands was not accounted for properly. The engineers used
density log porosity without correcting for the real matrix density.
If true, good engineering practice would have undersized all the
offshore equipment and early cash flow and rate of return on investment
would have been significantly reduced. If the myth that sandstone
is pure quartz is perpetuated, there will be more economic blunders
of this type.

Well logging literature is full
of other inconsistencies by mixing the names of minerals with
the names of rocks. Sometimes the words are synonymous,
sometimes not. For example:

MINERAL NAME ROCK NAME
Quartz
Sandstone
Calcite
Limestone
Dolomite
Dolostone
Illite
Shale
Check any service company chartbook and see how often the rock
names are used as mineral names or vice versa.

To
further confuse the uninitiated, many logs show data on a "porosity"
scale. These log curves are transforms of some measured physical
property to an approximate porosity, based on some arbitrary parameters.
Examples are density, neutron, or sonic porosity on so-called
Sandstone, Limestone, or Dolomite porosity scales. Porosity as
defined by these transforms is only directly useful if there is
no shale, the scale matches the rock mineralogy. and there are
no accessory minerals. Real reservoirs are rarely this simple.
DO NOT use these porosity transforms without further analysis
unless all the arbitrary assumptions used to create them match
exactly the rock you are analyzing.

Some
people call these porosity curves an “interpretation”.
They are not. They are merely a transform of the raw data to a
more attractive scale. The difference between a transform and
an interpretation is critical. Interpretation infers some intelligent
thought went into creating and understanding the result. The service
company running the log does not provide interpretations. YOU
are the interpreter.

There
are endless cases where a transform to an inappropriate porosity
scale has caused millions in losses due to poorly informed analysts
who see “gas cross over” when there is no gas, or
who read porosity directly from the transform and either seriously
over estimate or under estimate reservoir effective porosity.

In
spite of these comments, a number of charts and tables in this
Chapter and elsewhere in this Handbook show the word "sandstone'
when they really should say "quartz". I have not edited
the charts and tables taken from common sources, such as service
company chart books, so the common usage of incorrect terminology
is repeated even here.

It
should be noted also that this book uses the term "matrix
rock" to mean the solid, non-shale portion of a porous or
non-porous rock. In petrographic descriptions, "matrix"
is the clay between rock grains.